Monolayers of polarized kidney tubule epithelial cells form a selective barrier fundamental to homeostasis, achieved in part by structural and functional polarity of the plasma membrane. Membrane proteins with specialized functions are targeted to apical or basolateral surfaces by intrinsic sorting signals. The overall goal of this proposal is to understand the biogenesis and polarized sorting of three related plasma membrane proteins that play important roles in renal genesis and tubular cell proliferation: the membrane-bound precursor of the polypeptide growth factor EGF (preproEGF), which is expressed on apical surfaces of epithelial cells in the thick ascending limb of Henle (TALH) and distal convoluted tubule; the membrane-bound precursor of a structurally related polypeptide, transforming growth factor alpha (proTGFalpha), which appears to be expressed on apical membranes of collecting tubule cells; and the cellular receptor for EGF and TGFalpha (EGFR), which is found on basolateral surfaces of most renal tubule segments. Asymmetric localization of ligand and receptor may help explain why renal epithelial cells are normally quiescent in vivo except during tubular damage. PreproEGF, TGFalpha, and EGFR surface polarity may also influence kidney organogenesis, since their expression is tightly regulated during development. It has recently been shown that surface polarity of some membrane proteins is altered in certain pathophysiological conditions. There are reports, for example, that EGFR polarity is partially reversed in cystic epithelia from patients with autosomal dominant polycystic kidney disease (ADPKD). If true, then EGF/TGFalpha signalling could be profoundly influenced by asymmetric expression of other cellular components in ADPKD epithelial cells, depending on where EGFR is expressed. The specific aims of this proposal will delineate intracellular trafficking of newly synthesized preproEGF, proTGFalpha, and EGFR in renal tubular epithelial cells; identify intrinsic sorting signals for preproEGF, proTGFalpha and EGFR in renal tubular epithelial cells; and determine the effect of regulated posttranslational Ser/Thr phosphorylation on EGFR trafficking in renal tubular epithelial cells, with an emphasis on post-endocytotic sorting pathways.